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Friends have much in common -- including their DNA, study finds

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The bonds of friendship may be based on many things, and now the findings of a new study published July 14, 2014, in the Proceedings of the National Academy of Sciences suggest genetic similarities could be a factor.
(Visit Greenwich/Flickr)

In fact, the study found that the similarity between friends is like that shared between fourth cousins, or the people you share a great-great-great-grandparent with. It was also greater than similarities you expect to find based on shared ethnicity, the researchers said.

The study involved about 1,900 participants in a Massachusetts health study for which researchers had information on who was close friends with whom. About 1,300 pairs of close friends and 1.2 million pairs of strangers were identified based on the original participant group, which included people mostly of Irish and Italian descent (caveat: the lack of ethnic diversity among participants is a limitation of the study).

Researchers then looked at almost 500,000 sites within the DNA codes of those friends and strangers.

Among the between-friends similarities they found was that genes affecting the sense of smell were much more likely to be similar among friends.

What makes the study's findings particularly interesting is that it could contribute to the body of work surrounding evolution and altruism.

In evolutionary biology, hypotheses abound that seek to explain altruistic behaviors, e.g. risking one's own life and limb for others. Often these hypotheses focus on the genetic similarities between those being altruistic and those benefiting from the behavior. Kinship of some kind often comes into play, as does the idea that the individual who makes the altruistic sacrifice somehow increases his or her evolutionary fitness by helping closely related individuals to survive.

In other words, helping a relative (or genetically similar individual) to survive can mean some of the genes you share will survive into the next generation via your kin, even if you do not make long enough to pass some of your own on yourself.

So, the study's findings could lead to new and potentially interesting research around altruism and questions like this: Can friends detect this similarity by some mechanism? Does that mechanism or a similar one explain how genetically similar spouses find one another? Does it relate to a mechanism of kin or self recognition?

And then there is the possibility that detection mechanisms might have nothing to do with it, and similar friends are coming together in other ways.

Study co-author James Fowler of the University of California at San Diego told the Associated Press that similar genes may spur people to seek out similar environments, where they have a chance to meet, or the genes may lead to shared skills that become valuable when people work together.